The present application relates to the following co-pending United States Patents and Patent Applications: "Raster Output Scanner for a Multistation Printing System," U.S. Pat. No. 5,243,359, filed Dec. 19, 1991; "Diode Laser Multiple Output Scanning System," Ser. No. 07/948,531 (still pending), filed Sep. 22, 1992; "Multi-Beam, Orthogonally-Polarized Emitting Monolithic Quantum Well Lasers," Ser. No. 07/948,524 (still pending), filed Sep. 22, 1992; "Polarization Switchable Quantum Well Laser," Ser. No. 07/948,522 (still pending), filed Sep. 22, 1992; and "A Raster Output Scanner for a Single Pass Printing System which Separates Plural Laser Beams by Wavelength and Polarization," Ser. No. 07/948,530 (still pending), filed Sep. 22, 1992. These applications are all assigned to the assignee hereof and are all hereby incorporated by reference.
In xerographic printing (also called electrophotographic printing), a latent image is formed on a charged photoreceptor, usually by raster sweeping a modulated laser beam across the photoreceptor. The latent image is then used to create a permanent image by transferring and fusing toner that was electrostatically attracted to the latent image onto a recording medium.
While xerographic printing has been successful, problems arise when attempting to print at very high speed. One set of problems relates to the sweeping of the laser beam across the photoreceptor. As printing speed increases, it becomes more and more difficult to sweep the laser beam as fast as is required. While other sweeping methods are known, the most common method is to deflect the laser beam from a rotating mirror. Thus one way of increasing the sweep speed is to rotate the mirror faster. However, extremely fast mirror rotation requires an expensive drive motor, expensive bearings, and a powerful laser.
Other techniques of increasing the beam sweep speed include 1) sweeping the laser beam with a multifaceted, rotating polygon mirror, and/or 2) sweeping several laser beams simultaneously. Rotating polygon mirrors and their related optics are so common that they are generically referred to as ROSs (Raster Output Scanners). Printers that sweep several beams simultaneously are commonly referred to as multiple beam printers.
The beam sweep speed problem becomes very important when printing in color at high speed. This is because a color xerographic printer overlaps separate images for each color, called a system color, that is printed. While a dual color printer uses only two images, a full color printer typically requires four images: one for each of the three primary colors of cyan, magenta, yellow, and an additional image for black.
Color prints are currently produced by sequentially transferring and fusing overlapped system colors onto a single recording medium that is passed multiple times, once for each system color, through the printer. Such printers are commonly referred to as multiple pass printer. Conceptually it is possible to imprint multiple colors on a recording medium in one pass through the printer by using a sequence of xerographic stations, one for each system color. If each station is associated with a separate photoreceptor, the printer is referred to as a multistation printer; if the stations use different positions on the same photoreceptor, the printer is referred to as a single station/multiposition printer.
Multistation and single station/multiposition printers have greater printed page output than multipass printers operating at the same raster sweep speed. However, the commercial introduction of multistation and single station/multiposition printers has been delayed by 1) cost problems, at least partially related to the cost of multiple xerographic stations, each of which has its own ROS, and 2) image quality problems, at least partially related to the difficulty of producing separate images on each photoreceptor and then registering (overlapping) the separate images to produce a color output.
Proposed prior art multistation printers usually use an individual ROS (each comprised of a separate polygon mirror, lens system, and related optical components) for each station. For example, U.S. Pat. No. 4,847,642 to Murayama et al. involves such a system. Problems with such systems include the high cost of producing nearly identical multiple ROSs and the difficulty of registering the system colors.
A partial solution to the problems of multistation xerographic systems with individual ROSs is disclosed in U.S. Pat. No. 4,591,903 to Kawamura et al. That patent, particularly with regards to FIG. 6, teaches a recording apparatus (printer) having multiple recording stations and multiple lens systems, but only one rotating polygon mirror. Thus, the cost of the system is reduced. However, differences in the lenses and mirror surfaces could still cause problems with the registration of the different latent images.
Another approach to overcoming the problems of multistation printers having individual ROSs is disclosed in U.S. Pat. No. 4,962,312 to Matuura, et al. That patent teaches spatially overlapping a plurality of beams using an optical beam combiner, deflecting the overlapped beams using a single polygon mirror, separating the deflected beams using an optical filter (and polarizers if more than two beams are used), and directing the separated beams onto associated photoreceptors. The advantage of overlapping the laser beams is a significant cost reduction since the ROS is shared. It is believed that a commercial embodiment of the apparatus disclosed in U.S. Pat. No. 4,962,312 would be rather complicated and expensive, especially if four system colors are to be printed. The use of optical beam combiners to overlap beams so that they have similar optical axes and similar sized spots is believed to be difficult, expensive, and time consuming.
One solution to the problems with the teachings of U.S. Pat. No. 4,962,312 is disclosed in co-pending U.S. Pat. No. 5,243,359, filed Dec. 19, 1991. That application provides a raster output scanning system employing a rotating polygon mirror that simultaneously deflects a plurality of clustered, dissimilar wavelength laser beams having common optical axes and substantially common origins from common mirror surface areas. The clustered beams are subsequently separated by a plurality of optical filters and are then directed onto associated photoreceptors of a multistation printer. However, economically feasible optical filters require that the dissimilar beams to be separated by a sufficiently large wavelength, about 50 nm. For example, U.S. Pat. No. 5,243,359 utilizes lasers emitting at 645,695,755, and 825 nm. Since laser emission from closely spaced laser sources over this wavelength span is not yet available using one semiconductor material system, practical systems need to integrate different semiconductor material systems, such as AlGaAs and AlGaInP. Additionally, the wide wavelength span necessitates that the photoreceptive surface(s) has adequate response over the span, which includes infrared portions of the optical spectrum. However, few photoreceptive surfaces respond well in the infrared.
A further improvement on the teachings of U.S. Pat. No. 4,962,312 is disclosed in co-pending U.S. patent application Ser. No. 07/948,531, filed Sep. 22, 1992. That application provides a raster output scanning (ROS) apparatus which simultaneously sweeps a plurality of orthogonally polarized and dissimilar wavelength laser beams having common optical axes from common mirror surface areas. The swept laser beams are subsequently separated by a combination of a polarized beam separator and a dichroic beam separator. The separated laser beams are subsequently directed onto associated photoreceptive regions of a single station/multiposition printer, or onto associated photoreceptors of a multistation printer. Similarly dimensioned and registered spots are readily obtained on all photoreceptive regions, beneficially by establishing a substantially similar optical path length for each laser beam. However, the economic feasibility of that system requires inexpensive polarized beam separators and inexpensive dichroic beam separators which adequately separate coaxial combinations of beams by reflecting one beam and transmitting the other.
As described in the preceding co-pending U.S. Patent application, a required propterty of these separators is that their transmission/reflection characteristics do not change substantially as the laser beams are scanned through angles as large as 15.degree. from the nominally incident direction. While such separators are known in the art, their fabrication is complex and adds cost to the apparatus. Furthermore, since the transmission/reflection characteristics of such separators depend on the wavelength of the incident laser beams, as described in the above application, their optical performance in an operating ROS will change as the wavelengths of the coaxial beams change, e.g. due to temperature variations of the laser or as a result of laser degradation.
Accordingly, there is a need for an improved apparatus that simultaneously deflects and separates multiple, orthogonally polarized and dissimilar wavelength laser beams having substantially common optical axes. The apparatus should produce similarly dimensioned spots that readily can be brought into registration.